A typical motor vehicle door is mounted in a door frame on the vehicle and is movable between open and closed positions. Usually the door is held in a closed position by the latching engagement between a spring-biased ratchet pivotally mounted inside the door latch and a U-shaped striker secured to the door frame. The ratchet is most often spring-biased toward the unlatched position to release the striker and is maintained in the latched position to hold the striker by a spring-biased pawl or other mechanical structure. The ratchet cannot pivot to release the striker until the pawl is moved.
The majority of these door latches are exclusively manually operated both to unlatch the door and to relatch the door. Typically, the manual release handles are provided on the inside and outside of the door to release the ratchet from the striker by moving the pawl so that the door can be opened. The door is closed and relatched by manually pivoting the door so that the ratchet impacts the striker with sufficient force to pivot the ratchet to the latched position against the spring force exerted by the ratchet spring.
It is often difficult, however, to completely close and latch manually latching vehicle doors on current model vehicles because the desire to reduce vehicle weight and to improve fuel economy has led engineers to design vehicles with relatively thin and lightweight doors. Often relatively hard door seals are used with these thin, lightweight doors to improve sealing around the door, particulary at high driving speeds. Because many vehicle doors are relatively lightweight and have relatively hard door seals, many vehicles doors often have insufficient internal energy when pushed closed to compress these hard door seals and fully pivot the ratchet to the latched position to latch the door.
Power assisted door latch assemblies have been developed to overcome the problems associated with latching doors with lightweight construction and hard door seals. Power assisted door latch assemblies allow low internal energy or soft closure of the lightweight doors without the need to slam the door even with the increased seal pressure that results from relatively hard door seals. Existing power associated door latch assemblies typically function to latch a vehicle door in one of two ways: 1) by forcing the ratchet to pivot in the closing direction after engagement with the striker or 2) by forcing the striker to move in a door-closing direction after the striker is fully engaged with the ratchet.
Use of either type of power associated door latch assembly decreases the noise associated with door closing and decreases the manual effort needed to completely close the door. Power assisted door latch assemblies are disclosed by Ishikawa (U.S. Pat. No. 4,986,579), Kobayahi (U.S. Pat. No. 5,273,324) and Dowling (U.S. Pat. No. 5,520,425). In Ishikawa, the door latch assembly includes an electric motor for rotating the spring-biased ratchet from the partially closed to the fully closed position, and an electric switch for activating and deactivating the electric motor. In Kobayashi, the door latch assembly includes a rotatable lever for rotating the ratchet plate into the fully closed position, an electric motor manipulating the lever, and a mechanical linkage extending between the lever and the ratchet plate for rotating the ratchet plate into the fully closed position. In Dowling, the door latch assembly includes a motor driven gear, and a flexible wire extending between the driven gear and the ratchet for rotating the ratchet from the partially closed to the fully closed position. However, Ishikawa requires a complex rotary actuator for operating the switch, whereas the mechanical link and the flexible wire used respectively in Kobayshia and Dowling inefficiently transfers mechanical torque between the electric motor and the ratchet plate.
Latch assemblies which provide both power assisted opening and power assisted closing are also in use. In these power assisted latch assemblies, the same source of power, typically an electric motor mounted within the vehicle door, is used both to open the latch and to close the latch. The mechanical locking mechanism and some door opening handles can be eliminated from the vehicle door when these latch assemblies are used.
Power assisted opening and closing latch assemblies are taught by Bernard (U.S. Pat. No. 4,664,430), Kleefeldt (U.S. Pat. No. 4,518,180) and Tamiya (U.S. Pat. No. 5,232,253). These types of latching assembly are often advantageously used with powered sliding vehicle doors in which the latch must be released before the power door opening mechanism can be actuated to open the door. Bernard uses a cylindrical ratchet plate and a disc rotatably mounted on a common shaft, a pawl pivotally mounted on the disc for engagement with the ratchet plate, and an electric screw drive for rotating the disc between an open latch position and a close latch position. Kleefeldt uses a motor-driven gear and a sliding toggle linkage mechanically coupled to the driven gear for opening and closing the ratchet Tamiya uses a rack-driven link for rotating the ratchet from the open position to the closed position, and a lever coupled to the link for releasing the pawl from the ratchet to allow the ratchet to be rotatably driven back to the open position. However, Bernard stresses the electric motor by using the motor as brake to retain the ratchet plate in the closed position, whereas the latching mechanisms taught by Kleefeldt and Tamiya may not be reliable in environments where the door is forcefully closed into the latch. Also, the sliding toggle linkage used by Kleefeldt limits the mechanical torque which is ultimately applied to the ratchet
Accordingly, there remains a need for a simple, cost-effective power-assisted door latch assembly which efficiently transfers torque from the electric motor to the ratchet Further, there remains a need for a power-assisted door latch assembly which limits the stress applied to the electric motor by the ratchet.